This invention relates to methods and apparatus for monitoring the vibration performance of rotating machines and, in particular, to methods and apparatus for determining vibration in variable speed rotating machines.
In order to better evaluate the performance of rotating machines or parts thereof, such as a shaft, it has been found desirable to represent their vibration characteristics in terms of magnitude and phase versus frequency, otherwise known as a power spectrum. In order to produce the power spectrum of vibration based data, uniform time interval data must be generally processed in a manner that converts the time based vibration data to frequency based data. This process is generally effective in the case where the rotational speed is constant; however, it is not generally applicable in its original form when the rotational speed is variable.
The classical approach used in the prior art to convert time-based vibration data from a fixed speed shaft to frequency-based data utilizes numerous hardware components. The hardware generally includes a sensor that monitors the rotational speed of the shaft, such as a tachometer; a generator that produces a fixed number of pulses for each revolution of the shaft; an acceleration, velocity or displacement sensor that generates the time-based vibration data; one or more analog filters that limit the aliasing errors associated with data sampling; and a sampling/analog-to-digital conversion circuit to convert the signal to digital form. In general, the time-based vibration data is sampled either synchronously or asynchronously with respect to the rotation of the shaft. In the prior art systems, the sampling done during a fixed shaft speed would provide signals useful in obtaining magnitude and phase measurements at the shaft once-per-revolution frequency. However, when the shaft speed is changing during the interval that the vibration signal is being sampled, the magnitude and phase measurements made with said sampled data are inaccurate.
The prior art utilizes, generally, three different methods to perform vibration measurements for variable speed shafts: analog tracking filters, switched capacitor tracking filters and digital tracking filters. The analog tracking filter multiplies the vibration signal by the once-per revolution (fundamental) shaft speed signal. Sum and difference frequencies result in accordance with well known principles. The difference frequency is extracted with a low pass filter. The output of the low-pass filter is then further processed to produce voltages proportional to vibration amplitude. These results are converted to digital form with a standard analog to digital converter. The aforementioned low-pass filter must have a wide bandwidth to minimize measurement inaccuracies due to the variability of the speed of the shaft. Therefore, the analog tracking filter method suffers from poor noise rejection due to the use of a wider than optimal pass-band which is required to maintain tracking during shaft speed changes.
The second method, the switched capacitor tracking filter, uses discrete time sampling techniques to synthesize stable high accuracy multi-pole filters. The cut-off frequencies of these filters are controlled with a clock. To implement the tracking aspect of the filter, the shaft speed is used to generate the switched capacitor filter clock which varies with shaft speed. The output of the filter is then further processed to extract the once-per-revolution vibration amplitude and phase angle. These results are converted to digital form with a standard analog to digital converter. Variations of this technique may use synthesized signals controlled by a computer based on measured tachometer speed. All of the switched capacitor tracking filter techniques suffer from time delays in responding to shaft speed variations, which results in mistracking when the shaft accelerates. Also, the tracking filter bandwidth is increased to tolerate shaft speed variations during the vibration measurement interval to compensate for poor dynamic performance in cut-off frequency modification. This approach results in non-optimal noise rejection due to the wider bandwidths required for dynamic tracking.
The third method, the digital tracking filter, uses digital processing techniques to implement the tracking filter through software. Digital processing techniques may be used to implement the equivalent analog filter approach or emulate the switched capacitor approach. Tracking can be implemented by re-calculating filter coefficients, heterodyning to DC by multiplying by a digital sinusoid, order tracking by sampling the vibration signal as an integer multiple of the once-per-revolution frequency, or order tracking by over-sampling and then selecting samples that correspond to an integer multiple of the once-per-revolution frequency. The digital tracking filter method suffers because, in order to insure tracking, the tracking filter bandwidths must be wide enough to contain the full range of speed variations encountered during sampling of the vibration data within its band. These wider bandwidths include, by necessity, energy outside of the fundamental shaft speed, and therefore increase the vibration measurement error by considering energy components of unwanted vibrations. Furthermore, the delay between measuring the rotation speed and estimating the frequency spectrum introduces mistracking errors similarly present in the two other processing techniques.
It is therefore an object of the invention to provide an improved digital tracking filter which overcomes problems found in prior digital tracking filters.
It is another object of the present invention to allow for accurate measurement of vibration amplitude of a rotating structure having non-uniform rotating speed.
A still further object of the present invention is to produce the most optimum vibration magnitude measurements for a given number of rotations of the structure, or a given time window.
A still further object of the present invention is to produce optimum vibration magnitude measurements for a given rotational speed variation.
A still further object of the present invention is to dynamically provide the minimum tracking bandwidth for the actual speed variation.